1. Field of the Invention
This invention relates to a car-mounted imaging apparatus and a car driving assistance apparatus using the car-mounted imaging apparatus.
2. Description of the Related Art
Frame rates (number of imaging frames per second, unit: f/s) of imaging apparatuses such as a CCD (Charged Coupled Device) cameras and CMOS (Complementary Metal Oxide Semiconductor) cameras are generally follow those of television broadcasting and are fixed to 60 f/s (in the case of the NTSC system) and 50 f/s (in the case of the PAL system). In the case of a progressive (line sequential readout) type, the frame rate is ½ of the respective frame rate. Transfer rates of information imaged are fixedly determined by an information quantity of images and the frame rate. In this specification, this fixed frame rate will be assumed to be 30 f/s (that is, the progressive type of the NTSC system) for the sake of explanation. Incidentally, NTSC is the abbreviation of National Television Standards Committee and PAL is the abbreviation of Phase Alternation by Line.
On the other hand, various driving assistance apparatus for achieving traffic safety have been mounted in recent years to vehicles such as automobiles (hereinafter merely referred to as “cars”) and their examples include an auto-cruise control system (ACC), a preceding car follow system, a collision prevention system and a pre-crush safety system.
These driving assistance apparatuses indispensably need an imaging apparatus that plays the role of “eye” of the system. Because the frame rate is fixed in the imaging apparatuses according to the prior art as described above, however, the imaging apparatus is not yet entire satisfactory for accurately imaging various obstacles such as those that are under a stationary state and those that move at a high speed.
In other words, to acquire an image having a high S/N ratio for stationary obstacles, it is preferred to elongate the imaging time per frame but the fixed frame rate cannot satisfy this requirement. For obstacle moving at a high speed, it is preferred to increase the number of imaging frames and to grasp from moment to moment the moving process of the obstacles, but the fixed frame rate cannot naturally satisfy the requirement. In practice, therefore, the imaging apparatus can correctly image only those obstacles that exhibit specific motion corresponding to the fixed frame rate.
The imaging apparatuses according to the prior art that are directed to eliminate these problems include the following two types. (1) To transfer a greater number of frames within a predetermined transfer rate, image signals of a high frame rate imaged by the imaging apparatus are passed through a “thin-out processing circuit” for executing frame thin-out processing and image signals having a desired frame rate are acquired within a predetermined transfer rate. (2) Clocks (that decide a horizontal/vertical driving cycle) to be applied to the imaging apparatus are changed to acquire image signals having a desired frame rate. (For detail of these imaging apparatuses, refer to patent reference 1, for example).
Patent Reference 1:
Japanese patent No. 3,129,599
However, the technologies of these imaging apparatuses of the prior art ((1), (2)) are not yet free from the following problems.    (1) The frame thin-out processing, that is, a processing for skipping a certain number of imaging frames, is executed to transfer the image signals having a high frame rate within a predetermined transfer rate. Therefore, the increase of overhead resulting from the thin-out processing is unavoidable, and image quality after the thin-out processing is deteriorated because the pixel information is lost to the extent corresponding to the frames thinned out.    (2) When the clock to be applied to the imaging apparatus is changed, the frame rate changes, too, in such a fashion as to follow the change of the clock. When the clock is merely changed, however, insufficiency of the charge build-up time occurs in the CCD cameras when the clock is set to a high value, for example, or the drop of the output voltage of a photo-diode occurs in the case of the CMOS cameras. In either case, the problem such as the drop of sensitivity and deterioration of the S/N ratio occurs inversely proportional to the frame rate.